Axial packages are used to encapsulate a wide variety of dies or chips. Many different types of axial packages are available, many of which have been standardized. For instance, DO-41 is a package commonly used to encapsulate large power diodes such as those used in rectifiers. Other examples of standardized axial packages, which are generally standardized by the JEDEC Solid State Technology Association, include DO-15, DO-201AD and P600.
FIGS. 1a and 1b show an example of a typical axial package 10. FIG. 1a shows the components being aligned for assembly and FIG. 1b shows the components after assembly. The package 10 includes a die or chip 15, solder wafers 20, and leads 25 which terminate on one end with lead heads 30. As shown, the chip is located between the lead heads 30 and joined together by the solder wafers 20 after undergoing a high temperature reflow process during which the solder is melted. After the components are joined together they undergo a molding process to encapsulate them within a cylindrical body formed from a molding compound.
The axial package design shown in FIGS. 1a and 1b has a number of disadvantages. For instance, as seen in FIG. 1a, the components need to be carefully loaded and aligned with one another. Due to the nature of this process it is difficult to adopt an automatic process for piece part stacking and thus a manual process is generally employed. Another disadvantage is that different size chips need leads with different size lead heads, which in turn require different size assemblies in which the components are stacked and aligned. A single size lead head cannot be used for different size chips because it should fully cover the chip to prevent it from mechanical damage. However, the lead head should not be oversized because that would cause alignment problems. For this same reason a single size assembly in which to align the components is not practical. Thus, every time a new chip size is selected, all the related components need to be changed. As a result a large inventory of lead heads and assemblies need to be maintained.
Yet another disadvantage of the current axial package design is that there may be poor solder wetting because a solder flux process cannot be used to join the components since they are too fragile to go through a flux cleaning process. Without the use of a flux, significant solder voids may arise. To overcome this problem the subassembly is often exposed to a reducing atmosphere in a reflow or vacuum furnace. However, control and maintenance of the furnace needs to be carefully managed. Moreover, the solder reflow process is a manual process in which there are many factors that will affect soldering quality. These factors include the temperature profile, gas atmosphere, and the number of sub-assemblies that are placed in the furnace at the same time. Since each of these factors can vary there may be lot-to-lot inconsistencies among the final devices.
Accordingly, it would be desirable to provide an axial package design that overcomes the aforementioned problems.